Hybrid vehicles powered by a plurality of types of drive sources, such as an internal combustion engine and a motor-generator, are now in practical use. Japanese Laid-Open Patent Publication No. 2006-258140 discloses a hybrid vehicle having a gear train unit integrally incorporating motor-generators, which are arranged in a power transmission path between an internal combustion engine and drive wheels.
FIG. 10 shows the configuration of the gear train unit described in the above publication. With reference to the drawing, the gear train unit includes three planetary gear mechanisms P1, P2, P3 and two motor-generators MG1, MG2. Each of the planetary gear mechanisms P1 to P3 includes a sun gear, a ring gear, and a carrier.
In the gear train unit, a first carrier 1c of a first planetary gear mechanism P1 is connected to an input shaft In, which is connected to the engine of the hybrid vehicle, in an integrally rotatable manner. A first motor-generator MG1 is connected to a first sun gear 1s of the first planetary gear mechanism P1 in an integrally rotatable manner. A second motor-generator MG2 is connected to a first ring gear 1r of the first planetary gear mechanism P1 in an integrally rotatable manner.
A second planetary gear mechanism P2 and a third planetary gear mechanism P3 are arranged adjacently at the side opposite from the engine with respect to the second motor-generator MG2. A second sun gear 2s of the second planetary gear mechanism P2 and a third sun gear 3s of the third planetary gear mechanism P3 are connected in an integrally rotatable manner. A second carrier 2c of the second planetary gear mechanism P2 and a third ring gear 3r of the third planetary gear mechanism P3 are connected in an integrally rotatable manner. The second sun gear 2s and the third sun gear 3s, which are connected in an integrally rotatable manner, are connected to the first ring gear 1r in an integrally rotatable manner. The second carrier 2c and the third ring gear 3r, which are connected in an integrally rotatable manner, are connected to an output shaft Out of the gear train unit.
The gear train unit also includes a brake B, which is capable of stopping rotation of the second ring gear 2r, and a clutch C, which selectively connects and disconnects the first carrier 1c to and from the third carrier 3c. By switching the engagement state of the brake B and the clutch C, the power transmission of the gear train unit is switched among three modes.
In the gear train unit with the motor-generators of FIG. 10, skidding, or, in other words, dragging, occurs between the brake B and the second ring gear 2r when the brake B is engaged. This lowers the efficiency of the power transmission. Accordingly, as long as the conventional gear train unit with the motor-generators has the brake B, decrease of the power transmission efficiency caused by skidding cannot be avoided.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-258140